1. Field of the Invention
The present invention relates to an X-ray computed tomographic apparatus.
2. Description of the Related Art
As is generally known, an X-ray computed tomographic apparatus is designed to obtain an image (tomographic image) by calculating (reconstructing) the X-ray absorption coefficient of a tissue such as an organ on the basis of the amount of X-rays absorbed in a subject to be examined as an index called a CT value with reference to the X-ray absorption coefficient of water.
A reconstructed image inevitably contains image noise. Image noise is typically defined with reference to a variation in the CT value of a homogeneous phantom image as a standard deviation, which is generally called an image SD. In order to make diagnosis by observing a reconstructed image, e.g., to discriminate a shadow on the image as noise or a tumor, the image SD unique to the image must be considered. For this purpose, as shown in FIG. 9, for example, the water-equivalent thickness of a subject to be examined is calculated from a scanogram taken in advance, and a tube current corresponding to a designated image SD is calculated from the water-equivalent thickness. In helical scan, as shown in FIGS. 10 and 11, tube currents are discretely calculated at intervals corresponding to a helical pitch along the body axis so as to maintain the designated image SD.
In contrast examination, however, since the X-ray absorption coefficient increases due to a contrast medium injected into the subject, the image SD increases, as shown in FIG. 11.